by Mark McGregor
BBC News, Manchester
Charities find it difficult to talk to girls inside massage parlours
The figures are startling.
An estimated 4,000 trafficked women are thought to be working in the British sex trade at any one time, having arrived from across the world.
The majority willingly travel here from eastern Europe, west Africa and south-east Asia with the promise of jobs, more money and a better way of life.
What they end up as are commodities. Coerced, often forced, to work as prostitutes, they are moved from address to address, city to city, by the gangs that control them.
They have little chance of escape, let alone the life they were promised, and live in constant fear of being "sold" on to another gang.
But according to those on the frontline trying to help these women, it is not a threat confined to foreign nationals - some believe it is a two way street.
In short: British women are being threatened with being sold abroad.
Peter Green is the project director of Barnabus, a Christian outreach which works with people involved in the sex trade in Manchester, dubbed "the sauna capital of Europe".
Although the charity finds it difficult to access the city's massage parlours, volunteers have seen a sharp rise in the number of women from eastern Europe working the streets in the last 12 months.
"I believe it (sex trafficking) is widespread," Mr Green told the BBC.
"But I also believe we are not aware this is a two way issue, that young women are being taken out of our country and sold abroad because they owe money to the massage parlours."
Although he cannot go into the specific details, Mr Green has been alerted to the threat hanging over one British woman by fellow charity workers - at a time when the UK is marking the 200th anniversary of the Parliamentary Act which brought about the abolition of the slave trade.
"It's something I'd never considered, but when I first heard about the plight of this young lassie it became a logic to me," he said.
"If people are being transported one way they can be transported the other way.
"This young lassie was having to service 20 guys a day just to pay off interest, that's before she started to earn anything, and she was under constant threat of being sent out - trafficked - out of the country."
After raising the issue at a meeting of the Stop the Traffik organisation in Manchester and discussing it with other Church of England charities, Mr Green believes it is an issue that has not been previously considered.
"I don't believe people see it, particularly the church," he said.
"We just see what we see on the television and in the newspapers (about trafficking) and we are working from that without looking in the opposite direction."
Mr Green has raised the issue with other charities
Figures about the influx of vulnerable women into this country, because of the covert nature of trafficking, are almost impossible to pin down accurately.
So when it comes to data supporting the concept of British women being forced out of the UK, the figures are non-existent - as yet.
The Association of Chief Police Officers (ACPO), believes the threat of being sold may well be a tool that gangs use to coerce women into sex work.
Grahame Maxwell, deputy chief of South Yorkshire Police and programme director at the new UK Human Trafficking Centre in Sheffield, told the BBC: "We are aware of internal trafficking, where foreign nationals are grooming English girls and taking them city to city.
"It is, however, very hard to get a person who doesn't want to leave the country to leave.
"So unless they are drugged and hidden in the back of a van, which then you are talking something very serious indeed, a lot of it may well be just threats."
But according to the National Christian Alliance on Prostitution (NCAP), there is "an issue" of UK women being trafficked abroad.
Spokesman Mark Wakeling told the BBC that Mr Green's was "not an isolated story".
"I have heard of it as well, from a group working in the West Midlands, about young girls being groomed by men who try to get them to go overseas to 'visit people'.
"Who knows what happens when they get there? It could be viewed as a form of slavery," he added.
But for Christian groups like NCAP and other charities that work with women involved in the sex trade, the issue of British women being groomed for sexual exploitation within the UK is just as pressing.
Mr Wakeling points to the United Nations (UN) definition of trafficking, which encompasses themes such as the abuse of power or a position of vulnerability as well as the recruitment and transfer of people.
"A lot of people think trafficking only happens when there is a change of country," he said.
"Actually, the vast majority of women on the streets are being exploited because of their vulnerability in the same way someone brought from Lithuania to Manchester is being exploited as well.
"You might not always have a physical enslavement (as a prostitute) in the UK but you certainly have a psychological enslavement."